Coded track circuit system of railway signaling



April 14, "1942. R. M. GILSON CODED TRACK CIRCUIT SYSTEM OF RAILWAY SIGNALING Filed Apr il s, 1941 3 Sheets .$heet 1 HIS A'ILTONEY April 14, 1942. R. M. GILSON 2,279,930

comab TRACK CIRCUIT SYSTEM OF RAILWAY SIGNALING Filed April '3, 1941 3 Sheets-Sheet 5 HIS ATTORNEY Patented Apr. 14, 1'942 UNITED STATES APATENT OFFICE min TRACK omom'rsYs'rEM or RAILWAY SIGNALING Robert M. Gilson, Pittsburgh, Pa., assignor to The Union Switch &-Signal Company, Swissvale, Pa.. a corporationofPennsylvania A plication Aprils, 1941 S rialNo. 386.642

' Claims. 246-33) My invention relates to coded track circuit sys-Y tems of railwaysignaling and it has special reference to -A. P. B. and other two-direction runining systems of this character wherein use is made of the coded feedback operating. principle that firstwas disclosed byFrank H. Nicholson Patent No. 2,021,944; (issued November 26, 1935), and that also is shown by numerous, later patents and applications of common ownership herewith. p a l 1 =10 Generally stated, the object of my invention is to increase the utility and broaden the range of application of two-direction running track circuit combinations of thecoded feed back class.

A more specific object is to. overcome a reset :5 difiiculty which is experienced with such combinations when they are included in A. P. B. systems of the combined masterfeed back special code energy type which Ralph R. Kemmerer application Serial No. 294,897 (filed September 14, 1939) and other later cases disclose and claim.

Another object is to provide improved means for insuring that the master andfeed back portions of the Kemmerer and other similar track circuit combinations will automatically reset and 5 synchronize themselves following restoration of Y those combinations to their normal vacant section conditions.

In practicing my invention I attain the above and other objects and advantages by supplementing the code supply apparatus at one end of each circuited section by a converter relay which during and immediately following certain occupied section conditions recurrently interchanges the on and off periods of the coded energy which 5 is impressed upon the section rails at the converter location. Upon restoration of the section to its normal vacant condition, this interchange disrupts any reset-preventing coincidence of the .master code-energy pulses and the opposing 4 t V andof westbound traffic into the section is re- (and still independently supplied) pulses of special code energy andthereby permits the master energy pulses to reach the master track relay at the feed back section end. In responding in the usual manner that relay now cliscontinues the special code energy, substitutes .off period pulses of .feed back energytherefonand thereby restores the track circuitto the desired coded feed backfform of vacant section operation.

I'shall describe two A, P. B. system track circuit combinations which embody my invention and shall then point out the novel features there- .of in claims. These illustrative embodiments are disclosed in the accompanying drawings in which: 1

Fig. 1 is a diagrammatic showing of the automatic reset. improvements of my invention applied to one of the coded feed back track circuit combinations of the earlier referred to Kemmerer application Serial No. 294,897;

Fig. 2 illustrates the various forms of coded copending Kemmerer-Staples application Serial No. 386,632 (filedApril 3, 1941) discloses and claims. v a Inthe several views :of the drawings like reference characters designate corresponding parts.

Referringfirst to Fig. 1, I have there reproduced in a form modified to utilize the present invention one of thetrack circuit combinations of the A. P. B. system which Kemmerer application Serial No. 294,897 discloses and claims. This reproduced track circuit combination is from Kemmerers-composite diagram of Fig. 1a le and it utilizes the basic track circuit structure of Kemmerers simplified diagram of Fig. 5. F

Track circuit combination of Fig. 1 The track "section to whichis circuit is applied forms part of the usual siding-to-siding A.1P. B. stretch (not completely shown in Fig. 1) of twodirection running track I2and it includes the portion ofthat stretch which is between'insulated rail joints 3 at intermediate locations II and III.

This section is equipped with the usual signals SEII and SWIII by which the entry of eastbound spectively governed. These two signals, in turn,

.arecontrolled through the medium of two coded of the earlier mentioned Nicholson Patent; No.

Whe n organized into a complete A. P. B. system of'the non-line-wire character which the earlier identified Kemmerer application disclosesand claims, these two reversely arranged track circuits, cooperate with corresponding facilities 'of adjoining trackwsections (not illustrated herein) in a mannerenabling each train that enters the A. P. B. stretch to establish. both directional and a following protection without the aid of line wires for signal control purposes. One of these track circuits transmits master code energy which controls the eastbound signal SEII and it will be referred to as the master or eastbound track circuit; the other transmits feed back code energy which controls the westbound signal SWIII and it will be referred to as the feed back or westbound track circuit.

As the Kemmerer application more fully explains, both of these track circuits include the rails l and 2 of section II-III; the eastbound or master circuit further includes a track transformer TTE and a code transmitter CT installed at the section east or masterfend III and aback circuit further includes a track transformer TTW and a coding contact 9 (of therelay TR) located at the section west or feed back end II and'a pair of code following track relays TW-and TW I installed at the section east end.

In this basic Kemmerer combination, energy for operating the master or eastbound track circuit is derived from the terminals BX-CX of an alternating current energy source at the section east-end III. In the adaptedarrangement which Fig. 1 of the present application shows, these terminals are recurrently connected with transformer TTE by a contact 5 of a relay CTP which repeats the code producing operation of one or another of the contacts l8D-8EJ2E3L of the code transmitter CT at the relay location and which thus recurrently picks up and releases at oneor another of the code speeds of 180, 80 and 20 cycles per minute. As in Kemmerers original organization, selection of the code speeds is made by relays HE-PAES through the medium of contacts 263035.

In the adapted combination which Fig. 1 represents, those contacts form part of a driving circuit for the repeater relay CTP and they determine which of the code transmitter contacts l8'0-8il'2iL is included in that circuit. When (as Fig. 1 shows) the transmitter contact 139 isso included,'code repeater relay CTP is picked up 180 times each minute over a circuit that extends from th'e positive terminal of a local supply source through front contact 189, a contact IEO of arelay 'CCR (later to be described), conductor28, front contacts 26 and 30 of devicesHE and PA, conductor 25, and the winding of relay CTP back to the negative terminal of the supply source. 7

Each time that relay CTP is so picked up, contact 5 thereof connects transformer TTE with source BXCX and thereby causes the' section rails to be supplied with a pulse of master code energy which constitutes an on period in one of the master 180 codes of Fig.2. In releasing at the end of each of these on periods contact'fi breaks the transformer energizing circuit, interrupts the supply of master trackway energy, and thereby produces a master code "that relay topick up and release its contacts 1-'8-9 in step with the operations of coder contact 5 at the section east end. Under the vacant section conditions which are represented in Fig. 1, the frequency of. this pick-up and I release is 180 timesper minute; when, however,

'i'at the section west end II.

period of the received master code, this source relay CTP at location III operates at the slower code speed of times per minute, the vacant section operation of track relay TR then also occurs at that slower frequency; and when the control of relay CP is assigned to contact 20L, relay TR picks up and releases at the still slower code frequency of 20 times per minute.

Energy for operating the feed back or westbound track circuit is derived from terminals BK-CK-NK of an alternating current source During each off is connected with the track transformer TTW over the back point of contact 9 of the master track relay TR. In this manner there is produced a feed back code of the character which Fig. 2 represents and in which the on period pulses coincide with the off periods of the master code that is represented immediately thereabove.

At'time's (and as is shown in Fig. 2) the energy of this feed back code is of positive or given relative instantaneous polarity and then it is derived from terminals BKCK and applied 'to transformer T'IW over a circuit that includes front contact (ii of a device TP, conductor 63, back contact 9 of relay TR, front contact '64 of a device P and conductor 65. At other times the pulsed feed back energy is of negative or opposite relative instantaneous polarity and then it is derived from terminals NIL-CK and supplied to transformer TTW over a circuit that includes front contact 63 of a device WS, back contact SI of device TP, back contact 9 of relay TR and front contact 64 of device P.

This device P repeats the code following operations of the master track relay TR. It is picked up over contact 8 of that relay during each on period of the received master code, it has a period of "release delay which is slight- 1y less than the shortest master code 01f cried, and it terminates (by releasing contact 64) each feed back energy pulse prior to the beginning of a succeeding master code on period.

As the Kemmerer specification more fully explains, all of the given polarity pulses of the thus supplied feed back energy which reach the section east end III operate the positive polar "track relay TW thereat and allof the opposite polarity pulses which are received at the same location produc'e'code following response on the part of the companion or negative polar track relay TWI. In the adapted arrangement of Fig. 1', connection of both of these relays with the section rails is established over the back point of master coder contact 5. This connection includes track transformer T'I'E and conductors 404l, it is made during and only during the master code off periods, and it prevents the on period pulses of master code energy from reaching the feed back circuit track relays section to the west.

track sections in the A. P.B. stretch (not fully shown here) are controlled in customary automatic block manner. control scheme is of the three-indication variety and each track section therein makes use of decoding apparatuswhich: "(1) governs the indication of the eastbound signal SE for the section; and (2) selects the coding of the master track circuit energy that is Such eastbound decoding apparatus is provided at the west end of the section (see location II of Fig. l) andit there includes home and distanti decoding relays HE and DE which through frequency selective circuits DU receive pick up energy from a decoding transformer D'I. That transformer in turn, is excited by a direct current source, designated by the terminals plus andminus, over a circuitvvhich is controlled by a pole changing contact 1 of the master track relay TR.

Th elements just named cooperate (l) to cause both the home relay HE and the distant relay DE to pick up when the track relay is following master code of the 180 energy pulse per 'minute variety; (2) to allow only the home re- .lay HE to be picked up when the track relay is following the slower masterlcode of 80 pulses per minute; and (3) to causeboth of the relays HE and DE to release when the track relay either holds its contacts continuously in one position or follows the still slower master detection code of pulses per minute.

I in their determination of the aspect which the eastbound signal SEII displaysythese two decoding relays are aided by an auxiliary relayPA which is pickedup (over front contacts 8 and 36 of relaysTR -and P) during and only during codefollowing operation of relay TR andwhich "is sufficientlyslow in releasing that. it bridges the off periods of the system master codes. It (1) when the master contact 24.

1 The eastbound decoding relays PA and HE at i the section west end also select the code frequency at which the pulses of master energy are 'suppliedto the rails of the next track section to the west. As long; for example, as the master track relay TRat location III is receiving trackway energy of either th master 180 or the master 80 code pattern, contacts 26 and an of relays HE1PAjkeep' the driving circuit for master coder device CTP assigned to transmitter contact I869 andthereby cause the rails of section II--IlIto be supplied with master energy of the 180 pulse per minute code. In the event, however, that relay TR at location III fails to follow code, the driving circuit for coder relay C'I'P then .is assigned (if contact of directional stick "relay ES is picked up)- to transmitter contact 80and master energy of the 80 pulse per minute code is impressed upon section 11-111.

That eastbound signal supplied to the track X bound signals SE for the several consecutive bound signals SE in theusual automaticfblock system manner and when expanded into a completeA. P. 3. system, it thus provides the desired following protection for all eastbound train moves through the A. P. B. stretch,

The earlier described"feed back track circuit organization of Fig. 1 is suitable for expansion into a polar code system by which the westbound signals SW for a complete A. P. B. stretch likewise may be controlled in automatic block manner. As Fig. 1 shows, such a signal control system will be of the three-indication variety and eachtrack section therein will make use'of decoding apparatus which (1) governs the indication of the westbound signal SW for the section; and (2) selectsthe polarity of the feed backtrack circuit energy that is supplied to the track section to the east.

Such westbound decoding apparatus is provided at the east end of the section (see location III of Fig. 1) and it there includes three slow release relays DW, HW and TP which are controlled by the two feed back track relays 'IW and TWI. The first of these slow release relays DW is a distant decoding device and it is energized over a front contact 15 of the first track relay 'IW; the second slow release relay HW is a home decoding device and it is energized over a from; contact l6 of the second track relay 'I'Wl and the third slow release relay '1? is a code detecting device and it is governed by all four of the relays 'IW, I'W'I, DW and I-IW through the medium of contacts [5, I6, 38 and 39.

All of these contacts operate in circuits by way of which the windings of the named relays are energized from a local control source. Each of the relays 'IW and HW has a slowness of release sufficient to span the off periods of the feed back trackway code, while the code detecting rela TP has an even longerperiod of release delay,

When given polaritypulses of feed back energy are received from th section rails; the track relay TW responds while relay TWI remains inactive. In consequence, distant decoding re lay DW is picked up and at contact 38 sets up a circuit (including back contact 39) over which each release of relay TW supplies a pulse of pick-up current to relay TP, and thereby also holds that relay continuously picked up. .When, however, th received feed back energy is of the opposite polarity the negative polar track relay 'I'Wl responds and holds the home decoding relay I-IW continuously pickedup. "Contact 39 nowsets up a circuit (which includes back contact 38) over which relay 'I'P is held continuously picked up by the recurring releases of contact I6.

It will thus be evident that: (1) when neither of the feed back track relays 'IW and TW! is following codeall three of the relays DW, HW and TP are deenergized and hence released; (2)

1 when the positive polar track relay 'IW is re- The master track circuit organizatidn of Fig. 1 is, therefore, effective to control the eastsponding to coded energy relays DW and TP are recurrently energized and thereby caused to hold their contacts continuously picked up; and (3) when the negative polar track relay 'IWl is re sponding to coded energy relays HW and TP are similarly caused to hold their contacts continuously picked up.

In determining the aspect which is displayed by the associated westbound signal SWIII the justnamed relays make use of contacts 5|, 52 and 53 that are included in circuits by way of which lighting current is selectively supplied to the signal lamps G, Y and R. The clear lamp G receives lighting current over front contacts 5| and 53 when relays DW and TP are both picked up; the approach lamp Y receives lighting current over front contact 53, back contact 5| and front contact 52 when relays HW and TP are both picked up; and thestop lamp R receives lighting current over back contact 53 when relay TP is released.

These feedback code responsive relays DW, HW and T? at the section east end also cooperate to select the polarity of the feed back track circuit energy which is supplied to the west end of the track section that adjoins to the east. Referring to location II of Fig. 1, it will be seen that this polarity selection is effected by contact 6| of relay TP. When this contact is picked up, feed back transformer TTW is excited from the positive polarity source BK-CK; when, however, contact 6| is released (at a time when contact 63 of'relay WS is picked up) the feed back current track transformer is excited from the negative polarity sou1ce,NK-CK.

It will thus be evident that: (1) when either of the two feed back track relays TW and TWI is responding to received feed back energy, relay "1? will be picked up and the rails of the section to the east then will be supplied with positive polarity feed back energy; and (2) when neither of the westbound feed back track relays TW and TWI is following code, relay TP will be released and the rails of the section to the east then will receive negative polarity feed back energy.

As the Kemmerer specification fully explains: 1) a frequency code block control system of the earlier described master track circuit type governs the associated eastbound signals SE of an A. P, B. stretch in such manner as to provide following protection for all east-bound train moves through that stretch; and (2) a polar code block control system of the just described feed back track circuit type governs the associated westbound signals SW in such manner as to provide following protection for all westbound trainmoves through the A. P. B. stretch.

Further included in Kemmerers complete A. P. B. system are directional protection facilities through the medium of which advance of the leading vehicle of a train past the entering headblock location (I or IV of Kemmerers Fig. la-d) successively conditions the opposing track circuits ahead in such manner as to set the signals for opposing traffic at stop.

Aiding in this successive conditioning are the previously mentioned directional stick relays which Fig. 1 of this application shows at ES and WS. A set of these directional stick relays is provided at each of the two en ds II and III of the Fig. 1 track circuit and the relays ESWS in each of these sets are controlled in customary A. P. B. manner over local energizing circuits. Such energizing circuits are fully shown and described by the Kemmerer application and for that reason they have not been reproduced here.

From the Kemmerer application description of those circuits it will be seen that they func tion to keep both of the relays in each ES-WS set deenergized as long as the A. P. B. stretch of single track remains vacant. Under such vacant stretch conditions, therefore. all of these directional stick relays ES and WS stay releasedas Fig. 1 of the present application shows.

From the Kemmerer application description of directional protection during eastbound train moves through the A. P. B. stretch, it will be seen that the normally released eastbound directional relay ES at a given intermediate location picks up when the leading vehicle of an eastbound train first passes that location and so stays until the trailing vehicle of the train clears the east end of the section of which the given location marks the west end. In the case of relay ES at location III of Fig. 1, therefore, pick-up is produced by the advancement of an eastbound train past location III and is continued until that train has passed out of the east end of the section to the east of that location.

From the Kemmerer application description of westbound directional protection, it will similarly be seen that the westbound directional stick relay WS at any given intermediate location is picked up-only from the time that the leading vehicle of a westbound train first passes that location until the trailing vehicle of the train has cleared the west end of the track section to the immediate west of the location. In the case of relay WS at location III, therefore, pickup is produced by the advancement of a Westbound train past location III and is continued untilthat train has cleared location II. Upon such clearance, relay WS at location III immediately releases.

Each of these eastbound stick relays carries a conventional knockdown contact 35 which prevents the rails of the Westwardly extending section from receiving master code during the passage of westbound traflic through the A. P. B. stretch. As the Kemmerer application explains, this prevention contributes to the provision of westbound directional protection. Each westbound stick relay WS, similarly, carries a corresponding knockdown contact 68 which prevents the rails of the eastwardly extending section from receiving negative polarity feed back energy when eastbound traffic is passing through the-stretch.

Also carried by each of these eastbound stick relays ES is a contact 42 which is included in the pick-up circuit of the westbound decoding relays -SW-HWTP at the same location and which maintains those pick-up circuits intact only when relay ES is released. Upon pick-up of that relay, contact 42 disconnects all three of the relays DWI-IW-TP from their energizing source and thus renders them unresponsive to code following operation on the part of feed back track relays TW-TWI.

Further operated by each westbound directional stick relay WS is a corresponding contact I27 which performs a similar function in connection with the eastbound decoding relays HE-DE at the stick relay location. As long as relay WS is released this contact maintains decoding transformer DT connected with its excitmg source in the usual manner. When, however, relay WS picks up, contact I21 removes all exciting current from the transformer and thus renders the eastbound decoding relays HE-DE both unresponsive to code following operation on the part of contact I of the master track relay by which they are controlled.

When the thus far described portions of the track circuit structure of Fig. 1 are expanded into a complete A. P. B. system (see Figs. la-le of Kemmerer application Serial provide: (1) both directional and following protection control of the eastbound wayside signals SE; (2) both directional and followingprotection control for the westbound wayside signals No. 294,897) they SW; and (3) cab signal indication-control dur-- ing train moves through the circuitedsections in the direction of from west to east or towards the master end III of the track circuit of Fig. 1-.

For such control of train-carried cab signals during these eastbound or feed back to master end train movements, direct-use can, of course, be made of the frequency code pulses of master energy that originate-in the east end source BX-C'X for each section and that are applied to the section rails over front contact ofmaster coder CTP and through the east end track transformer TTE. Quite obviously thesemaster code pulses of alternating current trackway en- Use cannot be made of the feed back energy pulses which under vacant section conditions are supplied through transformer ITW (at the west end II of the Fig. 1 track section) for the reason that under occupied section conditions the master track relay TR. is continuously released and, the supply of these feed back pulses is thereby discontinued. Moreoven even were the supply to be continued the character of these pulses still would not be suitable for cab signal control since it has been seen that the. polarity rather than the frequency of recurrence varies with changing conditions of advance westbound traflic.

To provide for cab signal operation during westbound train moves, the complete A. P. B. system of the Kemmerer applicationutilizes aux-- iliary contacts X80 and Xl80which form a part of each and every one of the code transmitters CT in the A. P. B.system. Theseauxiliary contacts operate at such rates as respectively-to produce special control code cycles of 180and 80 energy pulses per minute and (as will become more evident presently) each set thereof is called upon to produce code only when a train approaches the set location by way of the section thatis to the immediate east thereof.

Thus, when the track section IIIII of Fig.

l (to the east of the transmitter CT at location quency selective character (see Fig. 6 of the Kemmerer application) This energy is derived either from terminals BKl- -CK. or from terminals NK|.- CK and in all instances it is transmitted transformer TTW. 4

the special cab signal energy pulses are supplied.

In the event that the track section tothe im-. mediate west of location IIis vacant, contact 90 is picked up and the special cab signal code then is made up of 180 energy pulses per minuterthat are transmitted to transformer TT'W from. supply terminal BKI and over special coding contact Xl80, conductor I08, front contact 90 of device TP, conductor I04, the back contacts 92 and 64 of devices PA and P, and conductor 65. When,

however, the section to the west of location II is occupied and contact 90 is released, the cab signal code then is made up of energy pulses per minute supplied to transformer TTW from terminal NKI and over special coding contact X80, front contact H4 of device WS, back contact of device TP and back contacts 02 and 64 of devices PA and P.

The reset problem to be solved In coded feed back track circuit combinations which are supplemented by cab signal energy supply facilities of the special type just described, difiiculty is encountered in promptly causing the feed back supply apparatus to come into action upon movement of a train out of the section. At the instant a train leaves. section IIIIIof Fig. 1,'for example, recurring pulses of master code energy are being supplied to the section rails at the east location III and other recurring pulses of special cab signal energy are also being supplied to the same rails at the section west end II. Under certain combinations (as, for example, when the train. does not advance out of either end of the section butleaves it at an intermediate point by way of a spur siding not shown) the east-end supplied pulses of master energy may have the character shown at master (back coding) in Fig. 2, while the west-end supplied pulses of cab signal energy may havethe directly coinciding character which is shown at special 180 in Fig.2.

It is true that the east end and the west end coding devices CT by which these master and special codes are produced operate independently of one another yet it also frequently happens that auxiilary contact XI80 of device. CT at location II may get and stay in exact synchronism with master code contact 1800f device CT at location III. In that event, quite obviously, the before assumed departure of the train by way of the section spur siding (not shown) will leave a condition wherein each pulse of east-to-west flowing master code energy is met and directly opposed by a pulse of west-to-east flowing special code energy.

As long as this condition exists none of the master energy pulses can reach the track relay TR at location II, that relay continues to stay released and contacts 64 and 92 of associated devices P and PA continue the supply of special cab signal energy. Such continuance, quite obviously, prevents the feed back energy apparatus from again being brought into action. As long,

As will be seen from'Fig. 1, the supply of this special cab signal energy takes place only when relay PA releases contact 92 in response to the entry of a train into section IIIII. In that event relay P also reflects the continuously released condition of the mastertrack relay TR- by allowing contact 64 to remain continuously released. Under those conditions contact 90. of

relay TP serves to selectthe frequency at which moreover, as'this opposing pulse interference continues, both the eastbound and the westbound signals SEII and. SWIII are falsely kept at stop even though the section IIIII is now complete- 1y vacant.

To make the illustration just cited more coni crete, let it be assumed that the train enters section IIIII. from. the east, proceeds partway therethrough in the westbound direction,

shown).- During the entire time that any portion of the train is within the section, the westbound directional stick relay WS at the east location III is held picked up, contact I21 keeps the decoding relay Hill? at the same location released, and released contact 35 of eastbound directional stick relay ES at the same location prevents the control of master coder relay CTP from being assigned to contact 813 of transmitter CT.

At the west location II, meanwhile, master track relay TR (now continuously released, causes the associated devices P and PA to assign the eX- citing circuit for transformer TTW to one of the contacts Xl88-X80 of the associated transmitter CT. Assuming that the section to the immediate west of location II is vacant, relay TP will now hold contact 90 picked up and thereby cause transmitter contact XIBU to determine the pulsing of the special cab signal energy which is supplied to the west end of section II-III. Under these conditions, then, the named special code pulses recur at the rate of 180 per minute.

When, now, the westbound train leaves the section by way of the spur siding (not shown), this 180 pulse per minute special cab signal energy continues to be supplied at location II. At location III, the westbound directional relay WS registers the departure of the train by releasing. The master track relay TR at that location is in the meantime following code and in consequence decoding relay I-IE picks up as soon as contact I21 connects track relay element 1 with the plus supply terminal. This action restores (at contact 26) the control of master coder relay CTP to contact I89 of transmitter CT, and thus causes master I80 code energy to be supplied to the section rails at location III.

As previously pointed out, contact XlBfl of the west end code transmitter CT may operate in exact synchronism with contact iii!) of the east end code transmitter CT and in that event a direct coincidence of the special cab signal pulses with the master code energy pulses will positively prevent reset of the track circuit and continue this. prevention until such time as the opposing end code transmitter contacts do fall out of step.

Kemmerer recognized this problem of track circuit reset and in the A. P. B. system of his application Serial No. 294,897 he proposed to solve it through the provision of special relays Z--ZA (not shown in the drawings of the present application) at the feed back end of each of the system sections. Each set of these relays periodically cuts off (at a contact 9lagain not here shown) the transmission of cab signal pulses and by thus mutilating the special code pattern assures that the vacant-section pulses of master code energy will reach and operate the section track relay TR. That operation, in turn, causes: (I) contact 92 of relay PA to remove all special cab signal pulses from the section rails; and (2) contact 64 of relay P to restore the feed back supply apparatus to its desired condition of vacant section operation.

The solution of this invention For solving the above stated problem of track circuit reset, I propose to use structure which operates on a somewhat different principle than does Kemmerers mutilated code reset apparatus that hasjust been described. This structure may supplement either the special pulse supply equipment at the feed back end of the section or the master code energy supply equipment at the master section end. At either location it takes the form of a code converter relay which upon departure of a train from the section recurrently interchanges the on and the off periods of the there supplied code and thus disrupts any reset preventing coincidence of the master code and the special energy pulses.

That disrupting action positively assures that the master code energy will reach the track relay TR at the feed back end of the section. By responding in the usual manner, that relay now discontinues the special cab signal energy, substitutes off period pulses of feed back energy therefor and thereby restores the track circuit to the desired coded feed back form of vacant section operation.

In the embodiment of my invention which is shown in Fig. 1, this code converter relay takes the form of a device OCR which is installed as shown at the master supply location III and which carries contacts Hill-ml. These contacts are interposed between the master coder relay CTP and the code transmitter CT by which that relay is driven. When released they connect the driving circuit for the master coder relay CTP with the back point of one or the other of the code transmitter contacts I and 8E) and when picked up they transfer this connection to the front point of one or the other of the two transmitter contacts just named.

As long as the track section IIIII remains vacant, the operating circuit for this converter relay CCR is kept interrupted at a back contact I02 of the westbound code detecting relay TP. Under that condition relay CCR maintains its contacts HJO-|0I either continuously released (as shown in Fig. l) or continuously picked up. In either event, quite obviously, the master coder relay CTP receives pulsed driving current over one or the other of code transmitter contacts l80-8il in the same unaltered manner as were device CCR to be omitted from the track circuit apparatus.

When, however, the rails of section IIIII become shunted, relay TP releases contact I92 and thereby completes the earlier referred to operating circuit for the converterrelay. As shown in Fig. 1, this circuit extends from the positive terminal of a local supply source through front contact 8!) of the code transmitter CT, back contact )2 of device TP, conductor I03 and a winding (see Fig. 3) of relay CCR back to the negative terminal of the supply source.

As a result of this completion the converter relay CCR. is now brought into operation. During that operation, as later references to Fig. 3 will make clear, the relay contacts ISO-40! repeatedly shift from the picked up to the released and from the released to the picked up position. For reasons which will become more evident presently, the intervals between shifts are somewhat longer than the time which the track circuit II-III requires to reset under conditions that initially are favorable. Preferably each interval is of the order of several seconds.

From the diagram of Fig. 1 it will be seen that the just described shifting action of contacts IUD-NH takes place whenever section II-III is occupied. That shifting action, in turn, repeatedly transfers the driving circuit (conductorsv 25-28) for the master coder relay CTP from the back to the front and from the front to the back point of whichever one of the code transmitter contacts I80 and 80 is in control of that circuit. Each of those transfers, in consemay take.

by Fig. 3. I

noid which comprises winding I20.

tion, as Fig. 3 represents.

quence, interchanges the on and the fofi periods of the master code energy which at location III is supplied to the section. rails over contact 5 of the repeater relay CTP.

. The character of "this interchange will best beunderstood upon reference to Fig. 2. Contrasting the upper and the lower portions of that drawing view, it will be seen that upon each transfer from back to front or from front to back contact coding the. on and the off; periods of the moster, code are directly interchanged.

The code converter relay of Fig. 3

. Before describing how this code period interchange is utilized to overcome the earlier explained difficulty of track circuit reset, consideration will first be givento one preferred form which the internal construction of relay CCR This preferred construction is shown In the atrangement which Fig. 3 illustrates, the two relaycontacts. I00-IOI are governed as to position by a solenoid which includes a winding I06. As long as this winding remains deenergized, contacts IIlll--IOI havethe released position represented. When, however, the winding becomes energized, the contacts are, picked up. I I

For controlling the energization of winding I06, the converter relay makes use of a pair of contacts I09- I I0 that are included in the winding energizing circuit in the manner shown.

These contacts. arecarried by a pair of leaf springs IIII l2 which are insulatedly supported at their lower ends and which press at their I upper ends rightwardly against a cam H6, This cam, which preferably is of insulating material,

is mounted on a notching wheel II1 which, under the action of a ratchet H8, is rotatable in and only in the clockwise direction.

Rotativemovement is imparted to the wheel by a pawl H9. This pawl isoperated by a soleing is includedin the converter operating circuit earlier traced.

During released conditions of the earlier de scribed contact I I02, accordingly, winding I20 is supplied with apulse of. energizing current each time that code transmitter contact 80 oc- During each of these off or winding-deemergized periods, pawlIIS occupies (due to gravity or a spring not shown) its lowermost posi- Each time, however, thatwinding I20receives a pulse of energizing current, the pawl is moved to its uppermost position and there held until the beginning of the next foff period. :Upon that beginning, the pawl returns to its lowermost position. As long,

That windsequence, the cam driving wheel is slowly stepped aroundin the clockwise direction whenever the operating circuit for the converter relay is completed at back contact I02. I

.When the wheel carried cam IIB has the position shownby Fig. 3, the tip of one of the four bumps thereon causes spring III to separate contact I09 from its companion contact H0. As, however, the wheel and cam are notched around to the point where the trailing tip of the active pump clears the end of spring III, that spring is freed and it then moves contact I09 to the right and against contact H0. This contact engagement continues until the next bump on the cam has been advanced to a position approaching that shown in Fig. 3. As a result of such advancement, the two spring carried contacts are again separated. It will thus be seen that during the notching rotation of the cam and wheel, contacts I09-I I0 are recurrently engaged and disengaged at intervals which typically will be of the order of several seconds each. Each engagement as well ,as each disengagement, moreover, occurs very quickly and in a snap acting manner.

snap contact I09 into positive engagement with contact H0.

Similarly each clearance by the tip of a cam bump of the end of spring H2 allows that spring to snap contact IIO quickly away from contact I05. I

During the periods that contacts I09--I I0 are disengaged, solenoid winding I05 is deenergized and contacts IIlil-I III of the converter relay CCR then occupy the released position, as Figs. 1 and 3 show. When, however, contacts l09--IIO are engaged, there is completed for solenoid winding I06 a local energizing circuit which may be traced from the positive supply terminal through contacts II0I09 and the windinglllfi back to the negative supply terminal. Under this condition,

of course, contacts I00l0l of the converter relay will occupy the picked up position.

circuit combinations of the Fig. 1 type require,

therefore, as the, operating. circuit contact I02 remains released, pawl I I9 is moved up and down at a. frequency of 80 times per minute.

From Fig. 3 it willbeseen that each of these upward movements of the pawl rotates the cam,

the intervals separating these shifts should, as already indicated, preferably be of the order of several seconds each. Through obvious changes in the proportioning of the Fig. 3 relay parts these intervals may, of course, very readily be either lengthened or shortened.

Operation of reset facilities of Fig. 1

When the just described code converter relay (ICE is applied to a coded track circuit combination in the manner which Fig. 1 represents it overcomes the earlier described difficulties of specification and further description thereof is, therefore, not believed to be essential.

Examining the complete track circuit combination of Fig. 1 it. will be seen that as long as the stretch of track of which the circuited section IIIII forms a part remains vacant, the

westbound code detector relay TP will hold contact I02 picked up and thus continue the code converter relay CCR. in its normally inactive state. Under that condition, as already pointed out, contacts Hit-It! of the converter relay are either continuously picked up or continuously released.

Assume, now, that the rails of section IIIII become shunted, either by a train coming into the section or for any other reason. This shunt drops the master trackrelay TR at location II and thereby brings into action the special cab signal energy supply apparatus at that location. If the section to the immediate west is vacant, 180 pulses per minute of special code energy now are impressed upon the rails of section IIIII at the west end II thereof. Atthe east end III of the section, meanwhile, this same rail shunt deenergizes both of the feed back track relays TW-TWl, and thereby releases westbound code detector relay I'P. Contact I02 of relay TP now completes the operating circuit for the converter relay OCR and thereby causes that relay to shift the position of its contacts Hill-[| every few seconds. Each of these shifts effects, in the manner earlier explained, an interchange in the on and the off periods of the east end supplied (over contact 5 of repeater relay CTP) master code.

Assume next that the shunt is removed from the rails of section IIIII by some method other than train movement out of one end of the section. The stated requirement might, for example, be met by passage of a train out of the section by way of a spur siding (not shown). At the instant of such shunt removal the pulses of west end supplied special code energy are met and opposed by the pulses ofeast end supplied master code energy.

Under the assumptions already made these west end supplied pulses of special code energy recur at the 180 code rate. Assuming that the section of track to the east of location III is now vacant, the east end supplied pulses of master energy also recur at the 180 code rate. Due to the action of converter relay OCR, moreover, the on and the off periods of this master code energy are interchanged every several seconds.

If at the instant of shunt removal the special cab signal pulses fall in step with the master code 01f periods, the master pulses get through to track relay TR at location II in the desired manner. In responding that relay discontinues (by picking up relay PA) the supply of special cab signal energy and substitutes off period pulses of feed back energy therefor. This .substitution constitutes a resetting of the track circuit and restores the desired coded feed back form of vacant section operation thereto.

Under that condition, of course, the feed back energy gets through to and operates one of the east end track relays TWTWI. In consequence, westbound detector relay TP now picks up contact 102 and thereby interrupts the operating circuit for the code converter relay CCR. As a result of that interruption the converter relay is restored to its normally inactive state.

In the event, however, that the pulses of master 180 code energy initially do coincide with the ciai opposing pulses of special 180 code energy (see the "master 180 back coding and the special 1.80 portions of Fig. 2) then reset of the track circuit cannot take place and relay TR at location II remains continuously deenergized. This reset prevention, however, persists only until he code converter relay CCP. at location III shifts its contacts tilt-45H to the opposing position.

By that shift, the master code on and off" periods are interchanged to a positioning (see master 180 front coding of Fig. 2) wherein the master on period pulses do fall within the off period intervalsof the opposing specode (again see Fig. 2). In consequence, the energy of those master pulses now gets through to and operates the master track relay TR at location II. That operation, in turn, initiates the before described series of actions by which the desired reset of track circuit II-III is effected.

The above explanation has taken into account only those conditions of the two-direction running track stretch wherein the opposing pulses of the master and special code energies recur at the 180 pulse per minute frequency. Under certain other conditions, quite obviously, the opposing pulses may at the instant of rail shunt removal be recurring at the code frequency.

Consider, for example, the following situation. A two engine (front and rear) train occupies and is completely Within the limits of section IIIII. Near its center this train is now uncoupled. Next, the east portion of the uncoupled train is moved eastwardly out of section II III and completely into the section to the east of location III. At the same time the West portion of the uncoupled train is moved westwardly out of section IIIII and completely into the section to the east of location II. Following these two movements out of section IIIII, another train comes into that section at some intermediate point, such as by way of a spur siding (not shown). Subsequently this other train leaves the section by way of the same spur siding.

While the section rails are shunted by this spur siding train, there is impressed thereon: (l) at the east location IIImaster code energy of the 80 pulse per minute frequency; and (2) at'the west location IIspecial code energy also of the 80 pulse per minute frequency. By reason, moreover, of the detector relay TP at location III now being released, the code converter relay CCR now interchanges the on and off periods of this master 80 code every few seconds.

If at the instant the spur-siding train leaves the section the pulses of this master 80 code energy happen to fall directly in step with the opposing pulses of special 80 code energy, all three of the track relays TRTWTWI remain continuously deenergized and the track circuit cannot reset. As in the case of clashing pulse per minute codes, however, this reset prevention continues only until the code converter relay CCR at location III next interchanges the master code on and off periods.

When that happens, the master pulses are brought into step with the special code off periods and the energy of those master pulses then does get through to and operate the master track relay TR at location II. That operation, in turn, initiates the before described series of actions by which. the desired reset circuit II-,III is effected.

Track circuit combination of Fig. From the foregoing description of the track of track circuit reset facilities of Figs. 1-3 it will be seen claims, but may be extended to track circuit combinations of the further coded feed back form which the diagram of-Fig. 4 represents.

That diagram reproduces portions of one of the track circuit combinations from an A. B. system which a copending Kemmerer-Staples application Serial No. 386,632 (filed April 3,

1941) discloses and claims. That Kemmerer- Staples system is broadly similar to the A. P. B. combination of the Kemmerer application but difiers therefrom by various changes both in systern arrangement and in individual element design. All such differences as relate to the present invention will, it is thought; become evident from the following description of the Fig. 4 track section II--III and theapparatus directly associated therewith. Q l

In the track circuit organization which Fig. 4

represents the master energy source BX-CX I is'again at the section east end III and itt'ransmitslonperiod pulses of master code energy to a master track relay TR. at the section west end; the feed back energy source BK-CKis at that west end II and it transmits-off period pulses of feed back energy to a feed back track relay 'IlFtA at that section, east end; the eastbound wayside signal SEII is controlled by the master track relay TR. through themedium of homeand distant decoding relays and DE; and the westbound wayside signal SWIII is controlled by the feed back track relay TRA through the medium of corresponding decoding relays HW and DW. l 1

Each set of these decoding relays HE-DE and HW-DW is supplied with pick-up energyover a, pole changing contact I of the controlling track relay and the decoding'relays of the set have re sponse characteristics comparable to those of the frequency code" system relays HE-DE of Fig. 1.

That is, both relaysin each set stay released when the controlling track relay (TR orTRA) fails to follow code; the home relay (HE or HW) picks up whenever the controlling track relay follows code of either a 75 ora 180 pulse per'minute frequency; and the distant relay (DE or DW) picks up only when the controlling track relay follows code of the 180 pulse per minute frequency.

This selective response by the distant? relays DE and DW is produced by frequency selective units IBUDU interposed in the relay energizing circuits in the usual manner. The necessary rectification of the current which the decoding transformers D'IE and BTW supply to the home relays HE and HW'is effected by track relay contacts Ill arranged as disclosed and tion IIIis a back contact I24 of the eastbound I signal SWIII is similarly controlled by east-end decoding relays HW-DW. In each instance the controlled signal shows:' (1 clear when the associated track relay (TR or TRA) follows code atthe 180 rate; (2) approach when the track devices HEP and HWP. These repeater relays haveshort periods of release delay and the pickup circuit for each includes a front contact I22 of theassociated decoding relay (HE orHW).

Further included in the pick-up circuit for the I eastbound repeater device HEP at the west location II is a back contact I23 of the westbound directional stick relay WS. When, in consequence, relay WS is picked up, this relay HEP stays continuously released regardless of the position of the eastbound decoding relay HE. When, however, relay WS is released, relay HEP repeats eachpick-up and each releasing action of the decoding relay HE by which it is controlled. l

Further included in the pick-up circuit for the westbound repeater deviCeI-IWP at the east locadirectionall stick relay ES. When, in consequence, relay ES is picked up, this relay HWP stays continuously released regardless of the position of the westbound decoding relay HW. When, however, relay ES is released, relay HWP repeats each pick-up and each releasing action of the decoding relayHW bywhich it is controlled. For coding the master energy with which track section 11-111 is supplied at its east end III, use is made of a pair of code transmitters ICT and T which are there installed in the manner shown. These transmitters are provided with contacts I30 and 75 which pick up and release at the respective frequencies of and '75 times per minute.

Operated by one or the other of these code transmitter contacts, over a local driving circuit, is a master coder relay CTP. This relay is provided with contacts 130-431 which, depending claimed by a copending application Serial No. I 210,744, filed May 28, 1938, by Frank H. Nicholupon which of the transmittercontacts-IM and 15 is included in its driving circuit, recurrently pick up and release at one orthe other of the two code frequencies earlier named.

I When picked up, these contacts I3ll-i3l connect the section rails with source BXCX and thereby produce a master code on period; when released these contacts transfer the rail connection to track relay TRA and thereby produce a master code off period. Included in both of the connections just named is the usual track transformer TTE.

Normally, as already indicated, the driving circuit for the master coder relay CTP at location III is assigned to one or the other of the code transmitter contacts l8015. This assignment is controlled by the eastbound decoding relay HE for the next section to the east. Acting through the repeater relay HEP at location III, that eastbound decoding relay (not shown in Fig. 4) utilizes a contact I35 of device HEP together with contacts l36l3l of associated devices ES and HWP to select between the transmitter contacts I80 and 15.

When (as shown at location III) the control of coder relay CTP is assigned (in the manner just stated) to transmitter IBlICT, that coder relay is caused to produce a master energy code consisting of 180 on period pulses per minute; when, however, the relay driving circuit is transferred to transmitter ICT, relay CTP is caused to produce a master energy cod-c consisting of 75 on period pulses per minute.

In flowing westwardly over the section rails to west location II, the recurring pulses of this master code energy (from source BKCX) are impressed upon the master track relay TR by way of transformer TTW, back contacts lac-m1 of an impulse relay RC, and a rectifier I42. Under vacant section conditions, accordingly,

this track relay TR is caused to pick up and release its contacts 'I-III in step with the operations of code producing contacts l3E3I3I of the east end relay CTP. By the associated decoding relays HE-DE that response is, in turn, translated into appropriate indication selections for the eastbound wayside signal SEII.

The master track circuit organization of Fig. 4 is, therefore, efiective to control the eastbound signals SE in the usual automatic block system manner and when expanded into a complete A. P. B. system it thus provides the desired following protection for all eastbound train moves through the A. P. B. stretch.

For contributing to a corresponding control of the westbound wayside signals SW use is made of the feed back track circuit apparatus that earlier was referred to as supplementing the master organization just described. As already stated, this apparatus includes the track relay TRA at the section east end III and the trackway source BKCK at the section west end II. During the off periods of the west-end received master code energy, the before mentioned impulse relay RC connects this source with the section rails over the front points of its contacts I40I4I and thereby supplies the rails with the pulsed feed back energy by which the east end track relay TRA is operated.

Both in function and in operation this impulse relay RC closely resembles a device which Herman G. Blosser Patent No. 2,174,255 (now the subject of reissue application Serial No. 358,- 319 which was filed September 25, 1940, now U. S. Reissue Patent 21,783 of April 29, 1941) discloses at IR in a coded feed back track circuit combination for single direction running. When this relay RC is deenergized contacts I40- MI thereof occupy the released position (shown in Fig. 4) wherein the section rails are connected with the master track relay TR; when,

however, suitable pick-up current is supplied to the relay these contacts pick up and transfer the rail connection to the feed back supply source BKCK.

Under vacant conditions of section IIIII, this pick-up current is derived from an output winding I44 of the decoding transformer DTE and transmitted to relay RC through a transformer I45. The primary of this transformer is connected with winding I44 at all times; the secondary of transformer I 45 is connected with thewinding of relay RC over a front contact I46 of eastbound decoding relay HE and either a front contact I47 of the west bound directional stick relay WS at location II or a front contact I48 of the westbound decoding repeater relay. HWP for the section to the west of that location.

Relay RC responds only to positive polarity pick-up energy (which makes the left terminal of its winding positive with respect to the right terminal) and each release of the master track relay TR causes decoding transformer DTE to induce in Winding I 44 a pulse of such positive polarity potential. When the just described secondary circuit for transformer I is complete, each of those positive pulses is transmitted to relay'RC and produces a momentary pick-up on the part of contacts I4DI4I thereof.

Such pick-up, as already pointed out, connects the section rails with the east end source BKCK over a circuit which includes the usual track transformer TTW at location II. This connection causes a short pulse of feed back energy to be supplied to those rails. Each of these pulses coincides, under the conditions just described, with an off period of the east end supplied master code and hence is transmitted over the section rails to the east location III.

There each of the thus transmitted pulses of feed back energy is impressed upon the feed back track relay TRA. Such impression is by way of transformer TTE, back contacts I30-I3I of device CTP and a rectifier I5I This east end track relay TRA is, in consequence, caused to pick up its contacts 1-I0 in step with the off period pick-ups of the west end impulse relay RC. By the associated decoding relays HWDW that response is, in turn, translated into appropriate indication selections for the westbound signal SWIII.

For the purpose of equalizing the pick-up and release times of the so operated feed back track relay TRA, its pick-up circuit is supplemented by a stick circuit which prolongs each of the relay pick-ups for the full duration of the off period during which the pick-up occurs. This stick circuit includes a back contact I53 of the master coder relay CTP and a front contact I54 of the track relay; it is supplied with energy over either a front contact I55 of the eastbound decoding relay repeater HEP for the next section to the east or the front contact I24 of the eastbound directional stick relay ES at location III; and it operates in the same manner as that disclosed and claimed by Patent No. 2,172,893 which issued to Edward U. Thomas on September 12, 1939.

Under other conditions (which will be made more evident presently) the west end relay RC operates not as an impulse device for the supply of feed back pulses during the master code off periods but instead as a repeater for one or another of a pair of code transmitters ISDCT and I5CT at location II. Under these other conditions the energy supplied from source BK- CK over front contacts I4III4I is modulated independently of the master code pulses from east location III. To designate this independently modulated energy the term special code will be employed.

During these just referred to other conditions, a driving circuit for relay RC is set up over one or the other of front contacts I41 and I48 of devices WS and HWP at location II, back contact I 45 of the eastbound decoding relay HE at the same location, and a contact I51 of the repeater device HEP for that eastbound decoding relay. This driving circuit has much in common with certain corresponding control paths that an application Serial No. 315,801, filed by Crawford E. Staples On January 26, 1940, now U. S. Patent 2,244,901, granted June 10, 1941, discloses for headblock location devices KR. It includes a selector contact I58 of the westbound decoding repeater relay HWP for the track section to the west of location II. When picked up (at a time when devices HE and HEPare both released.)

this selector contact assigns the driving circuit forrelay RC to the code transmitter [800T (contact I80) at thesame location; when released (under releasedconditions of devices HE-HEP) ittransfers the circuit connections to contact I ofthe code transmitter 150T. l I

The above mentioned relays ES and WS are directionalstick devices of the character ordinarily used in two-direction running systems of A. P. B. signal control. They may, for example, beidentical with the correspondingly identified relays ofthe track circuit organizatiorrg which is disclosed in Fig. 1. In that event one set of these relays ES- -WS will be provided at each of-the two ends II and IIIof the Fig. 4 track circuit andcontrol thereof will be over local energizing circuits equivalent to those which the earliernamed Kemmerer application Serial No. 294,897 shows and describes.

When arranged in"that customary A. P. B. manner, both of therelaysin each ES-WS set will be deenergized and stay released as shown in Fig. 4 of the present application) as long as the A. P. B. stretch of single track remains vacant. Uponmovement of aneastbound train through thestretch the normally released eastbound relay ES at a given intermediate location will pick up when the leading vehicle of the eastbound train first passes that location and will sostay until the trailing vehicle of the train clears the east end of the section of which the given location marks the west end. Similarly,

upon; movement of a westbound train through the stretch, the normally released westboundrelay WS" at agiven intermediate location willbe picked uponly from the time thatthe leading vehicle of the westbound. train first passes that location until the trailing vehicle of the train has cleared the west end of the tracksection to the immediate westof the location.

When the A. P. B. stretchof which the Fig. 4

supplied master code recur at the 180 frequency rate; that the feed back relay 'IRA at location III is by oiT period energy received from the section west end caused to followcode at the same rate; and that the associated decoding re- "lays HW--DW ordinarily would set the signal SWIII'at clear. For a purpose which need not here be considered, however, the pick-up circuit for distant relay DW is broken, under these conditions, by a contact I60 of the eastbound decoding relay repeater HEP at location 111.

Signal SWIII is, in consequence, normally held at approach. This holding, however, does not interfere with the, desired clear showing upon the approach of a westbound train. Uponsuch approach the eastbound decoding relay repeater HEP .(at location III) for the next section to the east releases and restores (at contact I60) the control of westbound decoding relay DW to track relay 'I'RA; The eastbound stick relay ES at locationIII also is released and in consequence the master coder relay CTPat the samelocation now is maintained continuously deenergized at acontact l6l (now released) of device HEP.

This deenergization keeps (at back contacts l30--|3| the east end track relay 'I'RA continuously connected with the rails of section II -III,

removes all master. code energy therefrom, effects the release of eastbound decoding relays HE-DE at location II, and there causes contacts I46 and I51 to assign the control of impulse relay RC to on-e or the other of the west end code transmitters I C'I' and l5CT. Under vacant conditions of the track sections to the west, the westbound decoding relay repeater HWP will be picked up and contact I58 thereof now will assign the impulse relay driving circuit to the code transmitter contact I80.

,This assignment causes relay RC recurrently to pick up at the frequency of times per minute and thus supplies section II-III with special code energy (from west end source BK-CK) in the form of pulses that recur at the frequency of 180 times per minute. In being received at the east location III these special code pulses operate track relay 'I'RA and thereby pick up both of the decoding relays HWDW. In consequence, the westbound signal SWIII now shows clear.

Should thetrack section to the west of locacondition of relays I-IE-DE at location II and.

there keeps (at contact I46) the driving circuit for the relay RC assigned to one orthe other of the code transmitters I8liCT15CT.

Meanwhile, the advance train in the section to the west of location II keeps relay HWP thereat released and causes the westbound stick relay 'WS at the same location to stay picked up.

Contact I58 of relay HWP now assigns (over front contact M! of device WS and back contacts I57 and I46 of devices I-IEP and HE) the impulse relay driving circuit to the code transmitter contact 15. This assignment causes relay RC to pick up at the frequency of"75 times per minute and thus supplies the section rails with special code energy (from west end source Eli -CK) of the '75 pulse per minute variety.

In being received at location III, this special 75 code energy operates track relay TRA and thereby causes westbound signal SWIII to show approach, as aforesaid.

Should, finally, the advance train be in section II-III, the westbound signal SWIII will display stop. This, of course, results from the usual deenergization of track relay TRA and the consequent release of hoth of the westbound decoding relays I-IWDW at location III.

The combined feed back and special code track circuit organization which has just been described in connection with Fig. 4 is, therefore, effective to control the westbound signals SW in the usual automatic block system manner and when expanded into a complete A. P. B. system it thus provides the desired following protection for all westbound train moves through theA. P. B. stretch.

Such a complete A. P. 3. system will, quite obviously, also oifer all of the usual features of directional protection signal control. Since these do not form a part of the present invention, detailed description thereof will not here be given. It may, instead, be said that: (1) in the provision of eastbound directional protection use is made of knockdown contacts [41-448 carried by the westbound stick and decoding repeater relays WS-TIWP and arranged to prevent the eastwardly extending sections from receiving feed back energy (from source BKCK) during the passage of eastbound trafiic through the stretch; and (2) in the provision of westbound directional protection use is made of other knockdown contacts I36 and I6! carried by the eastbound stick and decoding repeater relays ESHEP and arranged to prevent the Westwardly extending sections from receiving master '75 code energy (from source BXCX) during the passage of westbound trams through the stretch.

The reset problems of Fig. 4

The problems of reset which the track circuit combination of Fig. 4 presents are broadly similar to those earlier explained for the Fig. 1 combination. These problems arise out of the fact that under certain conditions of the circuited track section IIIII the east-end supplied (from source BX-CX) pulses of master code energy coincide with and are opposed by the west-end supplied (from source BXCX) pulses of special code energy.

As will become more evident presently, these problems of reset prevention for section IIIII of Fig. 4 may be classified under the following two headings: (A) prevention of initial reset in section IIIII due to a clashing of 180 pulse per minute master and special codes upon removal of shunt from the section rails; and (B) prevention of initial reset in section IIIII due to a clashing of '75 pulse per minute master and special codes upon removal of shunt from the section rails.

As an introduction to the conditions under which these problems (A) and (B) are experienced, it may be observed that a shunt placed across the rails of section IIIII continuously releases both the master and feed back track relays TR. and TRA; that this continuous release of the master track relay TR at location II transfers (at contacts M5 and I5! of devices HE--HEP) the control of the west end impulse relay RC from the track relay TR to one of the code transmitters |8iCTl5CT at the same 10 cation; and that this transfer causes relay EC to be driven by one of the named location II transmitters for the purpose of supplying (from source BK-CK) pulses of special code energy to the section west end. As long, quite obviously, as the section rails remain shunted the supply of these special code pulses continues.

Upon removal of the shunt from the rails, moreover, these recurring pulses of special code energy continue to be impressed upon the section West end II and hence are transmitted over the section rails toward the east location III. At that east location, meanwhile, the section rails also are supplied with recurring pulses of master code energy which flow over the rails toward the west location III. Due to the resulting clash of these oppositely flowing pulses, the master code energy may be prevented from getting through to and operating the west end track relay TR. and the special code energy may be prevented from getting through to and operating the east and track relay TRA.

This prevention, of course, is at its highest eifectiveness when the energy pulses of the two oppositely flowing codes recur at the same frequency and fall directly in step with each other. Under various dephased conditions or the opposing pulses that elfectiveness may decrease to the point of substantial disappearance. Such, for example, is the case upon: (1) ordinary clearance of the section east end III by an eastbound train; and (2) ordinary clearance of the section west end II by a westbound train. In each of these instances the energy pulses that are supplied to the train leaving end of the section recur at the code rate, the energy pulses that are supplied to the opposite end of the section recur at the code rate, and resetting is prompt because times frequently occur when the energy supplied to each end of the section can get through to and operate the track relay at the opposite end thereof.

The mechanics of such self-effected reset are as follows. Falling in step with the off periods of the west-to-east flowing special code, the eastto-west flowing pulses of master energy reach and operate west end track relay TR; that operation picks up eastbound decoding relay HE at location II; that pick-up transfers (at contact I45) the driving circuit for impulse relay RC from code transmitter ISOCT to winding I44 of decoding transformer DTE; that transfer causes the impulse relay RC to pick up upon each release of the track relay TR; and that action restores the track circuit combination to the desired coded feed back form of vacant section operation.

Under certain other conditions, however, the two clashing codes do have the same frequency of pulse recurrence and in that event reset of the track circuit may be objectionably delayed. One example of such other conditions is to be found in the 180 code clash initial reset problem (A) that was earlier named. One Way in which that problem may be produced is as follows.

With the track stretch initially vacant, a train comes into section II-III not by way of either end thereof but at an intermediate point therein from a spur siding (not shown). The section to the east of location III being vacant, the east end coder relay CTP is being driven by the east end transmitter IBUCT and the section rails now are receiving (from east end source BXCX) 180 pulses per minute of master code energy. The section to the West of location II also being vacant, west end impulse relay RC then is being driven by west end transmitter IBBCT and the section rails now also are receiving (from West end source BK-CK) 180 pulses per minute of special code energy.

Should the train now leave section IIIII not by passing out of either end thereof but by way of the same spur siding (not shown) over which it entered, the pulses of 180 code energy that are fed to the section from opposite ends will either: (1) interfit and allow both the west and the east end track relays TR and TRA to be energized in a manner which produces immediate reset; or (2) clash and prevent each of these track relays from receiving operating energy from the opposite end of the section. It is in the latter event that the first stated problem (A) occurs.

Reset difficulty also may be experienced with opposing codes of the 75 pulse per minute frequency. One example of such difiiculty is to be found in the '75 code clash initial reset problem (B) that was earlier namedJ That problem may be produced in the following manner. Section IIIII is initially occupied by a two-engine (forward and rear) train which is completely within the limits of the section; this train is uncoupled near its center; the east portion moves eastward- 1y into the sectiontothe east of location III; the west portion moves westwardly into the section to the west of location II; and another train then comes into and subsequently leaves the section at an intermediate point, as by way of a spur siding (not shown).

At the instant that this other train so leaves the siding the section rails are receiving: (1) '75 pulses per minute of master code energy at the east location III; and (2i) 75 pulses per minute of special code energy at the west location II.

' These opposing pulses of 75 code energy will,

quiteobviously, either: (1) interfit and allow the track circuit immediately to reset, as already explained, or; (2) clash and delay reset until the code transmitters IBCT'at the two nds of the section may of their own accord fall out of synchronism. It is in the latter event that the second stated problem (B) occurs.

The reset apparatus of Fig. 4

I To overcome the reset difficulties just described for section II--III of Fig. 4, I supplement the track circuit combination of that figure by reset apparatus which broadly is similarto that earlier described for the Fig. l combination. IIAS is illustratively shown in Fig. 4, this apparatus takes the form of a code converter relay CCR installed at the feed back supply location II and carrying a contact I which is included in the special code driving circuit (one or the other code transmittercontacts I80 and I5 and back contact I40 of device HE) for the impulse relay RC at the same location.

When this contact I00 is released (as shown) it connects the RC driving circuit with the back point of whichever of the two transmitter contacts I80 and I5 is included in that circuit; when, however, contact I00 is picked up it then I shifts the driving circuit to the front point of the selected transmitter contact. Each shift,

therefore, in the position of contact I00 causesthe impulse relay RC to interchange the on and the off periods of the special code ener y which is supplied to the West end of, section This code converter CCR of Fig, 4 may have operating characteristics which duplicate those of the correspondingly identified device of Figs. 1 and, 3. In that event contact I00 normally occupies one or theother of its two positions continuously. When, however, pulsed driving cur- I rent is supplied to winding I (see Fig. 3) of the relay, this contact I00 then shifts its position at intervals of every few seconds. I I

Supplying relay CCR of Fig. 4 with such pulsed driving current at proper times is an operating circuit which includes a contact 151: of the code transmitterlSCT at location II, a back contact I64 of the eastbound directional stick relay ES hat the same location, and a back contact I65 of the eastbound decoding relay HE for section II--III. As long, accordingly, as either of the I two last named relays is picked up, the converter relay CCR holds contact I00 in the same position continuously; when, however, both of those two devices become released, pulsed operating current is supplied over code transmitter contact 15m to relay CCR and contact I00 then is caused to shift its position every few seconds.

Consideration will now be given to the manner in which this device CCR of Fig. 4 functions to resetthe track circuit which thatfigure illusthe section west end II.

I CCR. at location II.

Under vacant conditions of that entirestretch all of the track circuit combinations therein are operating in the normal ,coded feed back manher; the eastbound directional stick relay ES for each of the. circuited sections is released; the eastbound decoding relay HE at each location is picked up; and the west end code converter relay CCR. for each section is, in consequence, maintained inactive. I I

Assume, now, that the rails of section II-III become shunted at a time when the section to the west. of location II is vacant, and when the section to the east of location III also is vacant. This shunt drops the master track relay. TR at location II, and, bytransferring (at back contacts I and I51) the driving circuit for impulse relay RC to code transmitter I80CTat that location, causes 180 pulses per minute of special code energy to be impressed upon The release of relay HE at location IIwhich produced that transfer causes contacts I to complete the operating circuit for the code converter relay CCR; That completion, in turn,

allows recurring pulses of driving current to reach (over the front point of code transmitter contact 15x) device CCR and thereby causes that device to shift the position of its contact I00 every fewseconds. Each of these shifts effects, in the manner earlier explained, an interchange in the onf and the off periods of the west end supplied (over contacts I40-I4I of impulse relay RC) special 180 code. I

At the east end III of the section, meanwhile,

the driving circuit for the master coder relay CTP continues to be assigned to code transmitter ICT and, in consequence, pulses per minute of master code energy now also are impressed upon the rails of section II-III.

Assume nextthat the rail shunt is removed from section IIIII bysome method other than train movement out of one end ofthe section.

The stated requirement might, for example, be

met by passage of a train vehicleout of the section by way of a spur siding (not shown). At the instant of such shunt removal the pulses of east end suppliedmaster .180 code energy are met and opposedby the pulses of west end supplied special 180 code energy.

If at that instant the master code pulses fall in step with the special code foff periods; the energy of those master pulses gets through to track relay 'I'R at location II in the desired manner. In responding that relay discontinues (by picking up relay HE) the supply of special code energy and substitutes off. period pulses of feed back energy therefor. This substitution constitutes a resetting of the track circuit and restores the desired coded feed back form of vacant section operation thereto. Under that I condition, of course, the now picked up relay HE then relay TR. at location II remains continuousthe section rails at ly deenergized and the track circuit is prevented from resetting. This reset prevention constitutes the earlierstated problem (A).

Such prevention persists, however, only until the code converter relay CCR at location II shifts the position of its contact IUD. By that shift the special code on and off periods are interchanged to a positioningwherein the master on period pulses do fall within the 01f period intervals of the opposing special code. In consequence, the energy of those master pulses now gets through to and operates the master track relay TR at location II. That operation, in turn, initiates the before described series of actions by which the desired reset of track circuit IIIII is effected.

Under certain other conditions the opposing trackway pulses in section IIIII may at the instant of rail shunt removal be recurring at the '75 code frequency and the earlier stated reset problem (13) then may be presented.

One situation of this kind has already been described as involving an original movement of train vehicles out of opposite ends of section IIIII that is followed by a reshunting of the section rails. During this reshunt those rails are supplied: (1) at the east location III with 75 pulses per minute of master code energy; and (2) at the west location II with '75 pulses per minute of special code energy. At intervals of every few seconds, moreover, this special code energy has its on and off periods interchanged by the converter relay CCR at location II.

If at the instant of reshunt removal from the section rails the pulses of the east end supplied master 75 code energy happen to fall directly in step with the opposing pulses of the west end supplied special code energy, both of the section track relays. TR and TRA fail to receive operating energy and the track circuit cannot reset. Such, reset prevention constitutes the earlier stated problem (B).

As in the case of the clashing 180 pulse per minute codes, however, this reset prevention continues only until the code converter relay CCR- at location II next interchanges the special code on and 01? periods. When that happens, the master pulses fall into. step with the special code off periods and the energy of those pulses then does get through to and operate the master track relay TR. at location II. That operation, in turn, initiates the before described series of action by which the. desired reset of track circuit IIIII is eifected.

Again examining the driving circuit for the code converter relay CCR. of Fig. 4, it will be noted that this circuit includes aback contact I64 of the eastbound directional stick relay ES at location II. The purpose of this contact is to prevent needless operation of relay CCR.

Without this contact 164, relay CCR would be brought into and continued in operation from the time that the leading vehicle of an eastbound train first passed location II until the trailing vehicle of that train cleared location III. Analysis of the Fig. 4 track circuit combination will show that such operation is unnecessary and to prevent it, the driving circuit for relay OCR is broken (at contact I65) upon each pick-up of relay ES.

Such pick-up, of course, occurs only when some portion of an eastbound train is within section IIIII. Upon movement of the train out of the section, evenat an intermediate point by way of a spur siding (not shown), relay ES at location II immediately picks up and thereby restores (at contact I54) the CCR relay operating circuit to the exclusive control of contact I65 of relay HE.

Summary Although the reset facilities of my invention are herein shown and described as being used with coded feed back track circuit combinations of two particular forms (Figs. 1 and 4), it will be obvious that such uses are illustrative rather than restrictive and that my new reset facilities will also have utility with and hence may be applied to control circuit combinations of still further forms wherein a condition of 0pposing code pulse coincidence is at times encountered.

From the foregoing, accordingly, it will be seen that I have made important contributions to coded track circuit systems of A. P. B. and other equivalent signaling. In particular I have increased the utility and broadened the range of application of two-direction running track circuit combinations of the coded feed back class; I have overcome a reset difficulty which is experienced with such combinations when they are included in A. P. B. systems of the combined "master-feed back-special code energy type which Ralph R. Kemmerer application Serial No. 294,897 and other later cases disclose and claim; and I have provided improved means for insuring that the master and the feed back portions of the Kemmerer and other similar track circuit combinations will automatically reset and synchronize themselves following restoration of those combinations to their normal vacant section conditions.

Although I have herein shown and described only a few forms of railway trafiic controlling apparatus embodying my invention, it is understood that various changes and modifications may be made therein within the scope of the appended claims without departing from the spirit and scope of my invention.

Having thus described my invention, what I claim is:

1. In combination, a stretch of two-direction running railway track, a track section included in said stretch, master code equipment located at a given end of said section and effective to supply the section rails with master code energy in the form of recurring on period pulses that are separated by off period intervals, feedback apparatus located at the opposite end of said section and operable when and only when said master code pulses are there received further to supply the section rails with feed back energy in the form of pulses that recur in step with the off periods of the received master code, supplemental equipment also at said opposite section end which comes into operation only when said feed back apparatus is maintained inactive by failure of said master pulses to be transmitted. to that opposite end and which then supplies the section rails with special code energy in the form of recurring pulses that have no regular time coincidence with the aforesaid master code 011 periods, and means effective during the continuance of said feed back apparatus inactivity for interchanging at time spaced intervals the on and the ofi periods of the saidcoded energy that is supplied at one of said two section ends whereby to disr-upt any vacant section coincidence of said special code and "master energy pulses that following removal of a shunt fromithe section rails may initially prevent those master pulses from reaching said opposite section end and there restoring saidfeed back apparatus to its normal condition of vacant section operation.

2. In combination, a stretch of two-direction running railway track, 'a track section included in said stretch, master code equipment located at a givenend of said section and effective to supply the section rails with master code energy in the form of recurring on period pulses that are separated by off period intervals, feed back apparatus located at the opposite end of said section and operable when and only when said master code'pulses are there received further to supply the section rails with feed back energy in the form of pulses that recur in step with the off periods ofthe received master code, sup- V plementalfequipment also at said opposite section end which comes into operation only when said feed back apparatus is maintained inactive by failure of said master pulses to be transmitted to that opposite end and which'then supplies the section rails with special code energy in the form of recurring pulses that have no regular time coincidence with the aforesaid master code off periods, and means efiective during the continuance of said special code energy supply for interchanging at time spaced intervals theon and the off periods of the said coded energy that is impressed upon the section rails at one of said two section ends whereby to disrupt any vacant section coincidence of said special code and master energy pulses that following removalof a. shunt from the sectionrails' may prevent those master pulses from reaching said opposite sec tionend and there restoring said feed back apparatusto its normal condition of vacant sec tion operation. r v I l 3. In combination, a stretch of two-direction running railway track; a track section included in said stretch, master code equipment located at a given end of said section and eifective to supply the section, rails with master code energy in the form of recurring fon period pulses thatare separated by off period intervals, feed back apparatus located at the opposite end of said section and operable whenfand only when said" master code pulses are there received furtobe transmitted to that opposite end and which.

then supplies the section rails with special code energy in the form of recurring pulses that have no regular time coincidence with the aforesaid master code off periods, and means effective spaced intervals the on and off periods of the said coded energy that is supplied at one of saidtwo section ends whereby to disrupt any vacant section coincidence of said special code and master energy pulses that following removal of a shunt from the section rails may initially prevent those master pulses from reaching said opposite section end and there restoring said feed backapparatus to its normal condition of vacant section operation. y

4. Incombination, a stretch of two-direction during the continuance of i said master pulse transmission failure for interchanging at time running railway track, a track section included in said stretch, master code equipment located at a given end of said section and effective to supply the section rails with master code energy in the form of recurring on period pulses that are separated by off period intervals, feed back apparatus located at the opposite end or said section and operable when and only when said master code pulses are there received further tosupply the section rails with feed back energyin the form of pulses that recur in step with the off periods of the received master code, supplemental equipment also at said opposite section end which comes into operation only when said feed backapparatus is maintained inactive bythe placement of a shunt across the section rails and which then supplies those rails with special code energy in the form of recurring pulses that have no regular time coincidence with the aforesaid master code offf periods, and means effective? during the supply of said special code pulses for interchanging at time spaced intervals the on and the off periods of the said coded energy that is impressed upon the section rails at one of said two section ends whereby to disrupt any coincidence of said specialand master'code energy pulses that following removal of said shunt from the section rails may initially prevent those master pulses from reaching said opposite section end and there restoring said feed back apparatus to its normal condition of unshunted section operation.

5. In combination, a stretch of two-direction running railway track, a track section included in said stretch, master code equipment located, at a given end of said section and effective to supply the section rails with master code energy in the form of recurring on period pulses that are separated by oif period intervals, feed back apparatus located at the opposite end of said section and operable when and only when said master code pulses are there received further to supply the section rails with feed back energy in the form of pulses that recur in step with the off periods of the received 'master code, supplemental equipment also at said opposite sec:- tion end which comes into operation only when said feed back apparatus is maintainedinactive by failure of said master pulses to be transmitted to that opposite end and which then supplies the section rails with special code energy in the form of recurring pulses that have no regular time coincidence withthe aforesaid master code off periods, and means located at said givensection end and effective during the continuance of said feed back apparatus inactivity for interchanging at time spaced intervals the on and the off periods of saidthere supplied master code energy whereby to disrupt any vacant section coincidence of said special code and master energy pulses that following removal of a shunt from the section rails may initially prevent those,

master pulses from reaching said opposite section end and there restoring said feed back apparatus to its normal condition of vacant section operation.

6. In combination, a stretch of two-direction running railway track, a track section included in said stretch, master code equipment located at a given endof said section and effective to supply the section rails with master code energy in l rails with special code energy in the form of recurring pulses that have no regular time coincidence with the aforesaid master code off periods, and means located at said opposite section end and effective during the continuance of said feed back apparatus inactivity for inter- V changing at time spaced intervals the on and the off periods of said there supplied special code energy whereby to disrupt any vacant section coincidence of said special code and master energy pulses that following removal of a shunt from the section rails may initially prevent those master pulses from reaching said opposite sec tion end and there restoring said feed back apparatus to its normal condition of vacant section operation.

'7. In combination, a stretch of two-direction running railway track, a track section included in said stretch, master code equipment located at a given end of said section and effective to supply the section rails with master code energy in the form of recurring on period pulses that are separated by 01f period intervals, feed back apparatus located at the opposite end of said section and operable when and only when said master code pulses are there received further to supply the section rails with feed back energy in the form of pulses that recur in step with the off periods of the received master code, supplemental equipment also at said opposite section end which comes into operation only when said feed back apparatus is maintained inactive by failure of said master pulses to be transmitted to that opposite end and which then supplies the section rails with special code energy in the form or recurring pulses that have no regular time coincidence with the aforesaid master code 01? periods, a code converter relay at one of said two section ends which normally is inactive but which picks up and releases at time spaced intervals whenever said fed back apparatus fails to operate, and means responsive to each pick-up and to each release of said converter relay for interchanging the on and the off periods of the said coded energy that is supplied at said one section end whereby to disrupt any vacant section coincidence of said special code and master energy pulses that following removal of a shunt from the section rails may initially prevent those master pulses from reaching said opposite section end and there restoring said feed back apparatus to its normal condition of vacant section operation.

8. In combination, a stretch of two-direction running railway track, a track section included in said stretch, master equipment at a given end of said section operated by a master coding contact over a driving connection therewith and effective to supply the section rails with master code energy in the form. of recurring on period pulses that are separated by off period intervals, feed back apparatus at the opposite end of said section driven by the there received pulses of said master code energy and operable during and only during said master pulse reception further to supply the section rails with feed back energy in the form of pulses that recur in step with the oil periods of the received master code, supplemental equipment also at said opposite sec-.- tion end operated by an independent coding con,- tact over a driving connection therewith and effective only when said feed back apparatus is inactive still further to supply the section rails with special code energy in the form of recurring pulses that have no regular time coincidence with the aforesaid master code ofi periods, a code converter relay at one of said two section ends which normally is inactive but which picks up and releases at time spaced intervals whenever said feed back apparatus fails to operate, and means responsive to each pick-up and to each release of said converter relay for shifting the said equipment driving connection at the converter relay location from a front to a back or from said back to said front point of the said coding contact which that connection includes whereby to interchange the on and the off periods of the said coded energy that said contact driven equipment supplies and thereby disrupt any vacant section coincidence of said special code and master energy pulses that following removal of a shunt from the section rails may initially prevent those master pulses from reaching said opposite section end and there restoring said feed back apparatus to its normal condition of vacant section operation.

9. In combination, a stretch of two-direction running railway track, a track section included in said stretch, master equipment at a given end of said section operated by a master coding contact over a driving connection therewith and effective to supply the section rails with master code energy in the form of recurring on period pulses that are separated by off period intervals, feed back apparatus at the opposite end of said section driven by the there received pulses of said master code energy and operable during and only during said master pulse reception further to supply the section rails with feed back energy in the form of pulses that recur in step with the off periods of the received master code, supplemental equipment also at said ope posite section end which is operable only when said feed back apparatus is inactive and which then supplies the section rails with special code energy in the form of recurring pulses that have no regular time coincidence with the aforesaid master code off periods, a code converter relay at said given section end which normally is inactive but which picks up and releases at time spaced intervals whenever said feed back apparatus fails to operate, and means responsive to each pick-up and to each release of said converter relay for shifting the said driving connection for said master equipment from a front to a back or from said back to said front point of said master coding contact whereby to interchange said master code on and off periods and thereby disrupt any vacant section coincidence of said special code and master energy pulses that following removal of a shunt from the section rails may initially prevent those master pulses from reaching said opposite section end and there restoring said feed back apparatus to its normal condition of vacant section operation.

10. In combination, a stretch of two-direction running railway track, a track section included in said stretch, master code equipment located at a given end of said section and effective to supply the section rails with master code energy in the form of recurring on" period pulses that are separated by ofi period intervals, feed back apparatus located at the opposite end of said section and operable when and only when said master codepulses are there received further to supply the section rails with feed back energy in the form of pulses that recur in step with the off periods of the received master code, supplemental equipment also at said opposite section end operated by -a special coding contact over a driving connection therewith and effective only when said feed back apparatus is inactive still further to supply the section rails with special c'ode energy in the form of recurring pulses that have no regular time coincidence with the aforesaid master code ofi periods, a code converter relay at said opposite section end which normally is inactive but which picks up and releases at time spaced intervals whenever said feed back apparatus fails to operate, and means responsive to each pick-up and to each release of said converter relay for shifting the said driving connection for said supplemental equipment from a front to a back or from said back to said front point of said special coding contact whereby to interchange said special code on and ofi periods and thereby disrupt any vacant section coincidence of said special code and master energy pulses that following removal of a shunt from the section rails may initially prevent those master pulses from reaching said opposite section and there restoring said feed back apparatus to its normal condition of vacant section operation.

ROBERT M. GILSON. 

